1. Field of the Invention
The present invention relates to an improved apparatus operable to isolate a load from vibration of a base device to which it is attached and more particularly to provide a novel pressurizing scheme for a system such as is described in a copending application of David A. Osterberg entitled Load Isolator Apparatus, filed Jan. 29, 1997, having Ser. No. 08/790,647 and which is assigned to the assignee of the present invention. In the copending application, a novel damping concept is described to provide a soft damped suspension system between the load and the vehicle in translational directions while providing a stiff damped suspension system between the load and the vehicle in rotational directions. The present invention improves the stability of the system and protects against damage.
2. Description of the Prior Art
The above referred to copending Osterberg application has utility in various fields including automotive, test machinery and the like, but for convenience, the copending application and the present application are described in the environment of a payload, such as a satellite, mounted on a vibratable base such as a launching vehicle. In the copending application it is explained that it is often difficult to support the payload at the center of gravity and accordingly it is normal to mount the end of the payload at the end of the launch vehicle. The mounting apparatus previously used includes elastic means such as springs or dampers so as to allow enough motion of the payload along the translational axes (i.e. the launch axis, and the two axes perpendicular thereto) to maintain the alignment of its inertial measurement units in the payload, such as gyros and accelerometers. However, because the mounting is at one end, small rotational movements at the bottom of the payload result in large translational movements at the top which is undesirable since there is a limited amount of "rattle space" (i.e. the space between the payload and the aerodynamic outer shell). Accordingly, it is desirable to soften the translational motions while stiffening the rotational motions.
In the prior art, the payload has been supported by independent spring/damper units, typically mounted at various angles to provide the proper stiffness in each degree-of-freedom. In such a configuration, each spring/damper unit operates independent of the others. Other approaches have been to distribute the stiffness and damping around the base of the payload. The rotational stiffness of these isolation systems are limited by the center-of-gravity offset of the payload and the diameter across the base(mounting circle) and, while changing the angles of the spring/damper units allows some freedom in selecting the proper stiffness, the results are limited.
The invention of the copending Osterberg application overcomes the problems of the prior art by cross coupling opposite damping elements, rather than having them operate independently, to provide a soft damped suspension in transition and stiff damped suspension in rotation. The invention also describes an accumulator connected to the cross coupling conduits to provide pressurization for the fluid in the conduits and to receive fluid excess due to thermal expansion. Providing preload pressure to the system is desirable to prevent cavitation in the system during dynamic motion. In order to prevent a softening of the rotational damping due to the flow of fluid into the accumulator when the pressure of the fluid increases due to rotational forces, the conduits to the accumulator were made much more restrictive than the conduits in cross coupling. This usually has the desired effect since pressure changes due to most rotational forces are much more rapid than pressure changes due to temperature changes, and no significant amount of fluid would flow to the accumulator during the rapid pressure changes produced by normal rotational forces but fluid would still flow to the accumulator during the much slower temperature changes. However, it is desirable that the system provide hard damping even when very slow rotational forces are encountered at near static conditions and under these circumstances, the restrictions in the lines to the accumulator would not prevent fluid flowing out of the cross connections to the accumulator which results in the softening of damping. While the accumulator could be removed entirely, (and in some cases an accumulator may not be necessary), for most applications, pressures due to extreme temperature changes or very large rotational forces or pressures due to temperature changes added to the pressure due to rotational forces could become high enough to cause structural damage to the system.